What is "Ba'al Peratzim"?
Rashi citing Targum Yonasan: It is the plain called Peratzim. Radak - he similarly translated "Ba'al Gad" (Yehoshua 11:17) as the plain called Gad.
The Pelishtim were in Emek Refa'im (verse 18). Why did David strike them in Ba'al Peratzim?
Radak (19): They were in Ba'al Peratzim; since there were so many, they spread until Emek Refa'im.
Why did he attribute it to Hashem?
Radak: I had few men with me, and I struck such a multitude! Surely Hashem struck them in front of me.
What is "k'Feretz Mayim"?
Rashi: It is like water that breaches (overflows) the banks.
Radak citing Targum Yonasan: Like breaking an earthenware Kli full of water.
Radak: It is like a flood of water breaches a fence.
